Transmission of supervisory signals in a time division multiplex system



United States Patent 3,349,190 TRANSMISSION OF SUPERVISORY SIGNALS IN ATIME DIVISION MULTIPLEX SYSTEM Glenn L. Richards, Webster, N.Y.,assignor, by mesne assignments, to Stromberg-Carlson Corporation,Rochester, N.Y., a corporation of Delaware Continuation of applicationSer. No. 268,011, Mar. 26, 1963. This application Dec. 19, 1966, Ser.No. 603,047 13 Claims. (Cl. 179-18) ABSTRACT OF THE DISCLOSURE Anautomatic communication system including a plurality of communicationcircuits selectively connectable to respective ends of a commoncommunication channel in a selected time slot of a repetitive time frameby a plurality of individual gating means, scanning means for actuatingin a prescribed sequence the gating means not associated with acommunication connection, wherein requests for service determined by acurrent detector coupled to the common communication channel which isterminated in a matched impedance periodically with actuation of saidscanning means, requests for service being detected by absence of asupervisory current signal in the channel in coincidence withapplication of a scanning signal to a selected gating means.

This application is a continuation of Ser. No. 268,011,

led Mar. 26, 1963 and now abandoned.

This invention relates in general to automatic communication systemsand, more particularly, to the transmission of supervisory signals in anautomatic communication system of the time division multiplex type.

Although the invention herein disclosed is suitable for more generalapplication, it is particularly adapted for use in communication systemshaving a plurality of communication circuits, any one of which may beselectively connected to any other in response to the transmission ofappropriate coded signals. A telephone system would be typical of such asystem and in a telephone system the cited coded signals would emanatefrom the calling one of the communication circuits and would beindicative of the desired one of the communication circuits. Inaddition, coded signals indicative of a request for service, answer, ordisconnect, may be transmitted from any one of the stations. Intelephone systems it is customary to designate all such signals assupervisory signals and, there'- fore, the coded signals transmittedfrom any of the com- It is a more specific object of this invention toprovide a new and improved automatic communication system of the timedivision multiplex type wherein a common communication channel isemployed to transmit both communication and supervisory signals.

In the illustrated embodiment of the present invention, there areemployed several circuits whose details and functions are well known tothose skilled in the art to which this invention pertains, and since thecited applications illustrate and describe the operation of thesetypical circuits, the details thereof will not be described herein. Inaddition, there are provided a few circuits having functions notincluded in the cited time division multiplex communication systems. Theprincipal new circuit is the highway current detector which is employedto detect the presence of supervisory signals on the highway and totransmit appropriate signals to other circuits as required. A similarcircuit is the matrix current detector which is employed to detect thepresence of multiplexing signals to the line circuit gates and toprovide appropriate signals to other circuits as required. Thefunctioning of these and other circuits will be described more fully inthe detailed description.

Further objects and advantages of the invention will become apparent asthe following description proceeds, and features of novelty whichcharacterize the invention will be pointed out in particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to theaccompanying drawing which illustrates the concept of the invention inblock diagram form.

It is to 'be understood that only the details which are necessary tounderstand the inventive concept described herein have been shown. Forexample, it is believed that the inclusion of the circuit details of thevarious components which are functionally designated would only tend tomask or obscure the inventive concept described herein. That is, thecircuit details have not been shown since they form no part of thepresent invention and since various combinations of circuit elements arereadily available and well known to those skilled in the art who 7 maywish to practice this invention.

munication stations described herein will 'be referred to henceforth assupervisory signals.

In the prior art communication systems of the time division multiplextype, it was customary to provide a common communication channel totransmit the voice or other communicating signals and to provide aseparate data channel for transmitting supervisory signals from thesignaling circuit to the common control circuits. The patent applicationof W. F. Bartlett et al., Ser. No. 27,888, filed May 9, 1960, now PatentNo. 3,066,192, and assigned to the same assignee as the presentapplication, discloses a time division multiplex switching systememploying separate transmission and signaling channels; and the patentapplication of Barrie Brightman, Ser. No. 45,342, filed July 26, 1960,now Patent No. 3,134,859 and also assigned to the same assignee as thepresent application, disr DETAILED DESCRIPTION It is believed that theconcept and operation of one embodiment of the invention can best beunderstood by considering the drawing together with the followingdetailed description.

In accordance with commonly accepted practice, the circuit terminalsthat are connected to ground potential are indicated by a series ofsuccessively shorter and parallel lines contained within an imaginarytriangle. In a similar manner, circuit terminals which are connected topotentials which are positive or negative with respect to ground aredesignated or respectively.

Two line circuits, and 200, are illustrated which are connected by thepairs of lines 181 and 182, and 281 and 282, to their respectivetelephone instruments (not shown), which may include the same type ofsupervisory signaling circuit which is customarily employed in a typicaltelephone subset. That is, the instruments to-be I 3 used to distinguishbetween the two line circuits illustrated.

When the telephone associated with line circuit 100 is lifted for thepurpose of originating a communication connection to another telephone,a D.C. current will be completed from the positive potential connectedto one terminal of the upper winding of relay 110, through the upperinductive winding thereof, the upper left winding of transformer 140 toline 181, over the line to the telephone, through the dial andhookswitch contacts back to line 182, thelower left Winding oftransformer 140 and the lower inductive winding of relay 110 to thenegative potential connected thereto. The flow ofyD.C; current throughthis circuit will operate relay 110 and cause contacts 111 of relay 110to close and, when dialing is commenced, contacts 111 will openinsubstantial synchronism with the mentioned dial contacts. That is,contacts 111 will be closed when there is a direct current connectionthrough the telephone from line 181 to line 182 and contacts 111 will beopen when the direct current connection through the telephone from line181 to line 182 is open. Or, in telephone parlance, contacts 111 willrepeat the on-. or off-hook supervision of the associated telephone. Ina similar manner, contacts 211 of relay 210 in line circuit 200-willrepeat the onor offh'ook supervision of the telephone associatedtherewith.

When the telephone associatedwith line circuit 100 is on-hook, that is,when contacts 111 are open, the bias resistor 150 will establish apositive potential at the upper terminalof the resonant capacitor 1300fthe filter, comprising capacitors 120 and 130 andv inductors 160 and,170. If desired, the magnitude of this positive potential may be usedto provide class of service information. Associated with line circuit100 is a voice and supervisory gate 300 which comprises two similarportions. The upper half may be considered the calling gate, while thelower half is thecalled gate. A similar gate 350 is associated with linecircuit. 200. It should be observed that a winding of a transformer inthe calling half. of each voice gate is connected to the calling matrixand line number store 410,iwhile a winding of a transformer invthecalled half of each voice gate is connected to the called matrix andline number store 400.

In a manner familiar to those skilled in the time division multiplexart, it is possible to generate a plurality of time slot pulses in arepetitive time frame and to selectively apply one or more of the pulsesfrom one or more of the framesto a desired circuit. The cited patentapplications illustrate typical time division multiplex equipment anddisclose additional details relating to the gen eration, application,and utilization of time slot pulses. For the present purposes it shouldbe understood that a time slot pulse, designated as a probe pulse, andin a given time slot of a repetitive time frame, is applied once eachtime frameover lead 412 to the calling matrix and line number store 410and to the highway terminating circuit 550. In a manner more fullydescribed in the cited patent applications, the probe pulses aresystematically and sequentially applied to each of the matrixcrosspoints in matrix 410 which are not identified as the crosspointsassociated with busy lines effectively scanning the line gate to detectrequests for services. The line number store portion of the callingmatrix in line number store 410 provides the necessary memory of whichof the lines are busy. The probe pulse is an interrogating pulse whichis applied to each matrix crosspoint to ascertain if the line associatedtherewith is originating a request-forservice signal. More specifically,the application of the time slot probe pulse to conductors 331 and 332,which comprise. the two conductors at one cross-point, will cause apulse to be passed through transformer 330 so that for that time slot alow impedance circuit exists. between points A and B of the diode voicegate comprising the four diodes 310. During the same time slot but in asubsequent time frame, the probe pulse may be applied to 4 leads 381 and382 to turn on the diode voice gate comprising diodes 360. In thismanner, the idle line circuits are systematically and sequentiallyinterrogated or scanned to determine if they are originating arequest-for-service sig nal.

If it is assumed that contacts 111 are open when the interrogation ismade, it will be seen that when the probe pulse turns on the callinghalf of the voice gate 300, a positive pulse from capacitor will bepassed through inductor through the voice gate from point A to point Bto the calling highway 520, through the lefthand winding of transformer510, the called highway 530, to the highway terminating circuit 550 andground.

The highway terminating circuit 550 is gated on during the time slot ofthe probe pulse as may be seen by the connection from lead .412 to thehighway terminating circuit 550. The connection of the. highwayterminating circuit 550 to the highway must be gated on and off in.order to prevent the highway terminating circuit from derogating voicesignals in other time slots. The highway terminating circuit 550 forms aterminating circuit so that an actual current will flow through thecircuit traced above in order that a pulse may be passed throughtransformer 510 to the highway current detector 500. The highwayterminating circuit willnormally have an impedance equal to thecharacteristic impedance of the highway. It should be noted that thecurrent detected by the highway current detector 500 is a pulsed D.C.current. Thus, each time an interrogating or probe pulse is applied toone of the calling matrix cross-points, a pulse of D.C. current will bedetected by the highway current detector if the associated line ison-hook, or idle.

When a telephone is taken off-hook, the relay 110 or 210 associatedtherewith will operate in the manner previously described and,therefore, the contacts 111 or 211 will close. If it is assumed that,the telephone associated with line circuit 100 is off-hook, contacts 111will be closed and a ground potential willbe connected to the :upperterminal of resonant capacitor 130. Therefore, when a probe pulse isapplied over leads 331 and 332 to the upper half of the voice gate 300,:no pulse of D.C. current will be passed to the highway and detected bythe highway current detector 500. However, the lack of current detectionby the current detector 500 is not sufficient proof that a line isinstituting a request for service. For example, the removal of the.physical line circuit 100 and the voice gate 300 would provide the sameapparent result as a request for service so far as the highway currentdetector 500 is concerned. Accordingly, a matrix current detector 420 isprovided which tests for and detects the current in the matrixcross-point wires 331 and 332.Transformer 425 passes the. matrixcrosspoint current to the matrix current detector 420. The diodes 415are provided to prevent feedback from one matrix point to another.

In summary, when a line is on-hook a, pulse of D.C. current will bedetected by both the matrix current de-.

tector 420 and the highway current detector 500, and the coincidence ofthese two signals will indicate that the line being interrogated is notoriginating a request-forservice signal. But, when a pulse of D.C.current is detected by the matrix current detector 420 and not by thehighway current detector 500, it will be an indication that theinterrogating line is initiating a request for service;

that is, that the interrogated line is off-hook. When the last-namedcondition prevails, a time slot pulse in the time, slot of the probepulse will be applied to leads 501 and 421 from the highway currentdetector 500 andthe matrixcurrent detector 420, respectively, to thetime slot allotter 630. The time slot allotter 630 will respond to thecoincident receipt of pulses on lines 501 and 421 by assigning an idletime slot to the line requesting service.

The line. number store 410 will mark the line busy in response to asignal on lead 631 to inhibit further interrogation of the line andthereafter the calling portion of the voice gate 300 will be pulsed ononce each time frame in a time slot assigned to the calling line. Themanner of assigning idle time slots to a calling line and of turning onthe voice gate in the calling line is more fully described in the citedpatent applications.

The time slot allotter 630 also provides a signal over lead 632 to thecontrol circuit 620. The control circuit 629 will pass a time slot pulsein the time slot assigned to the calling line over lead 621 to the tonecircuit 610, which passes dial tone over lead 611 to the highway in theassigned time slot. Thus, the tone circuit 610 and the calling linecircuit 100 will be gated on in the time slot assigned to the callingparty and dial tone will be heard by the calling party.

In response to the receipt of dial tone, the calling subscriber willoperate his dial which, it will be recalled, causes contacts 111 to opena number of times corresponding to the digit dialed. During dialing, theupper half of the voice gate 306 is being turned on once each time framein the assigned time slot. Because a time frame has a period ofapproximately one hundred microseconds while the dial contacts have anopen period of approximately fifty thousand microseconds per pulse, manytime slot pulses will be transmitted during each open interval of thecontacts 111. The highway current detector 560 will detect the firsttime slot pulse of the first digit and a signal will be passed over lead502 to the impulse analyzer 600, which will signal the control circuits620 which will thereupon stop connecting the dial tone circuit to thehighway in the calling time slot. At the same time, the control circuit620 will start to pass time slot pulses to the highway terminatingcircuit 550 to provide a termination now that the dial tone circuit hasbeen disconnected. Accordingly, each time the contacts 111 are opened, aseries of time slot pulses are passed through the voice gate 30% and thetransformer 510 to the highway terminating circuit 550, in the mannerpreviously described. The highway current detector 500 is able to detectthe series of time slot pulses and passes them over lead 502 to theimpulse analyzer 600. The impulse analyzer 609 analyzes the series ofpulses and the time interval separating the series of pulses and therebyrecognizes the signals as digits and passes the digital information overlead 601 to control circuits 620. When the complete called number isregistered, the control circuit 620 passes the identity of the calledline over lead 623 to the called matrix and line number store 40! whichthen marks the called number as busy and, assuming the called line isthe one connected to line circuit 200, applies time slot pulses in thetime slot assigned to the calling line to leads 391 and 392 to turn onthe bottom half of the gate 350 in the assigned time slot. The foregoingdescription, of course, assumes that the called line is idle. Had thecalled line been busy, the called matrix and line number store 400 wouldhave returned a busy signal on lead 623 and the control circuit wouldhave controlled the tone circuit 610 over lead 621 to cause busy tone tobe applied to the highway in the assigned time slot. However, if it isassumed that the called line is idle and that low level tone ringing isemployed, the operation will proceed as described and the controlcircuit 620 will cause the tone circuit 610 to apply ringback tone andringing to the highway in the assigned time slot, but in alternate timeframes. During the time that the ringback tone and ringing signals areapplied to the highway, the calling voice gate will be gated open onlywhile the ringback tone is applied to the highway and the called voicegate will be gated on only while the ringing signal is applied to thehighway. While the ringing and ringback signals are being gated to thehihway, the highway terminating circuit 550 is not required and will notbe pulsed on. But during the interval between ringing, i.e., the silentperiod, the highway terminating circuit 550 will be connected to thehighway in the assigned time slot. The control circuits control thesefunctions over leads 621 and 623.

When the called party responds to the ringing of the called telephone byanswering, the relay 210 will be operated and contacts 211 will beclosed. The closure of contacts 211 will cause capacitor 230 to beshunted and, therefore, supervisory signals will no longer be sent fromline circuit 200 via the lower half of the voice gate 350 and throughthe left half of the transformer 510 of the highway current detector 500to the highway terminating circuit 550 and ground. The highway currentdetector 500 senses the cessation of the supervisory signals and passesa signal over lead 502 to the impulse analyzer 600 which, in turn,passes a signal over the lead 601 to the control circuits to stop theapplication of ringing and ringback tone and to start gating on theupper and lower halves of the voice gates 300 and 350, respectively, inthe assigned time slot of each time frame so that two-way voicecommunication may take place between the calling and called subscribers.In addition, the control circuit 620 passes a pulse over lead 622 toprevent the highway terminating circuit 550 from being connected to thehighway during the assigned time slot. It should be recalled that theterminating circuit is not connected to the highway While tones are bingapplied as at that time the tone circuits 610 provide the requiredtermination.

During conversation, the contacts 111 and 211 are closed and nosupervisory pulses of DC. current are applied to the line to be detectedby the highway current detector 500. The upper half, or calling half, ofvoice gate 300 and the lower half, or called half, of the voice gate 350are pulsed on each frame during the time slot assigned the calling lineand two-way communication takes places on a time division multiplexbasis.

Should either party operate the hookswitch, the DC. loop to that partysrelay will be opened and the contacts 111 or 211, as the case may be,will open. If it is assumed that the calling party operated hishookswitch, then contacts 111 will open and a pulse of DC. current willbe applied to the highway and detected by the highway current detector500. In this case, the highway terminating circuit 550 is not needed toprovide a terminating circuit for the supervisory signal as the calledline circuit 2410 provides a termination. I

It should be noted that the highway current detector 500 includes meansfor distinguishing between supervisory signals from the called andcalling line circuits. If the supervisory signals from the calling linecircuit are assumed to cause a current to flow therefrom, it will beseen that such current passes through the left winding of transformer510 from top to bottom, while a similar current from the called linecircuit will cause a current to flow from bottom to top in the winding.Accordingly, polarities of a first and second sense are induced in theright-hand winding of transformer 510 in response to supervisory signalsfrom the calling and called line circuits and, therefore, the highwaycurrent detector can determine whether a detected supervisory signaloriginated from a calling or a called line circuit. It may be seen thatthe same situation prevails if line circuit 200 is the calling line andline circuit is the called line. In this case, the calling or upper halfof the voice gate 350 will be turned on, thereby connecting line circuit200 to the calling highway 520, while the lower half or called half ofthe voice gate 360 will be turned on, thereby connecting line circuit100 to the called highway 530.

In the event that both the calling and called parties should attempt totransmit supervisory signals at the same instant, the currents willcancel out and no signal will be detected by the highway currentdetector 500. The highway voltage detector 560, however, will sense thechange in highway potential from the presence of the two simultaneoussignals and the highway voltage detector will pass a signal over lead562 to the highway current detector 500 which then provides appropriatesignals to the impulse analyzer 600.

When the calling and called parties disconnect, the contacts 111 and.211 will be opened and supervisory signals will, therefore, betransmitted to the highway. The impulse analyzer will analyze thesupervisory signals and determine that they represent on-hooksupervision and an appropriate signal will be transmitted over lead 601to the control circuit 620. The control circuits, in turn, will transmita signal over lead 623 to both the calling and called matrix and linenumber stores where the line busy marking will be erased so that thelines may be periodically interrogated in the time slot of the probepulse.

While there has been described what is considered at present to be thepreferred, embodiment of the invention, modifications thereto willreadily occur to those skilled in the art. It is not desired, therefore,that the invention be limited to the embodiment shown and described, andit is intended to cover in the appended claims all such modifications asfall within the true spirit and scope of the invention.

I claim:

1. In an automatic communication system; a plurality of communicationcircuits for originating and terminating communication connections, acommunication channel common to said plurality of communicationcircuits, a plurality of individual gating means each selectivelyconnecting a respective communication circuit to said channel uponactuation thereof, scanning means for actuating said gating means in aprescribed sequence, supervisory signal detector means coupled to saidchannel for detecting the presence and absence of supervisory signals inthe form of current signals of given magnitude generated by saidcommunication circuits, analyzer means connected to said detector meansfor indicating the terminating communication circuit designated by saidsupervisory signals, a tone signal circuit and control means responsiveto said detector means detecting absence of said supervisory signals forconnecting said tone signal circuit to the associated originatingcommunication circuit and responsive to said analyzer means forsimultaneously actuating the gating means associated with theoriginating communication circuit transmitting said supervisory signalsand the terminating communicationcircuit designated by said supervisorysignals periodically in a prescribed time slot of a repetitivecommunication time frame, said detector means including achannelterminating circuit selectively connected to and disconnectedfrom said channel in response to said scanning .means and said controlmeans to provide a terminating circuit for said supervisory signalswithout interfering with voice signals in other time slots, saidterminating circuit having an impedance substantially equal to thecharacteristic impedance of said channel.

2.. The combination as set forth in claim 1 wherein supervisory currentfrom the originating and terminating communication circuits flows infirst and second different directions in said channel, said detectormeans including sensing means for determining the direction of thedetected supervisory signals in said channel.

3. The combination as set forth in claim 2 further including allottingmeans for allotting a different time position of the repetitive timeframe to each originating communication circuit in response to detectionby said detector means of the absence of a supervisory signaltransmitted by said originating communication circuit in another timeposition.

4. The combination as set forth in claim 3 wherein said allotting meansis connected to said scanning means to prevent operation of the gatingmeans of the originating communication circuit by said scanning means inthe prescribed scanning sequence upon actuation by said detector meansso that subsequent supervisory signals from said originating circuit aretransmitted over said channel in said assigned time position.

5. The combination as set forth in claim 1 wherein said scanning meansincludes interrogating means for actuating said gating means in arepetitive sequential manner, off-hook detector means in eachcommunlcation circuit for transmitting a supervisory signal to saidchanmeans in coincidence with detection of absence of receipt of asupervisory signal from the communication circuit associated with saidgating means.

6. The combination as set forth in claim 5 wherein said gating meansconnected to each communication circuit includes a calling bridge and acalled bridge, said interrogating means being connected to the callingbridge of each gating means.

7. The combination as set forth in claim 5 wherein said off-hookdetector means includes capacitive means for storing a potential charge,a voltage source connected to said capacitive means, and switch meansfor discharging the potential charge stored in said capacitive means inresponse to detection of an off-hook condition in said communicationcircuit.

8. The combination as set forth in claim 5 wherein said control meansincludes a voltage detector coupled to said channel for providing acharacteristic signal in controlof said supervisory signal detectormeans when supervisory signals are transmitted simultaneously fromdifferent communication circuits.

9. In an automatic communication system, a plurality of communicationcircuits for originating and terminating communication connections, acommunication channel common to said plurality of communicationcircuits, each communication circuit including supervisory signal generating means for providing supervisory current pulse signals of givenmagnitude and a supervisory signal gate for connecting the associatedcommunication circuit to said communication channel, signal probingmeans for periodically interrogating each of said supervisory signalgates, communication channel current detector means responsive to saidsupervisory current pulse signals for.

eratedin response to the interrogation of each of said supervisorysignal gates and for generating a second control signal upon detectionof a signal from said supervisory signal gate, timing means forallotting a specific time slot of a repetitive communication time frameto each of said plurality of communication circuits only uponsubstantially coincident reception of said first control signal and saidsecond control signal, control means responsive to said first controlsignal and said second control signal for actuating the supervisorysignal gates associated with the originating communicating circuittransmitting said supervisory signal and the terminating communicationcircuit designated by said supervisory signals periodically in saidspecific time. slot.

10. The combination as set forth in claim 9 wherein the supervisorysignals from the originating and terminating communication circuit flowsin first and second directions in said channel, said supervisory signalshaving one polarity when generated by the originating communicationcircuit and an opposite polarity when generated bythe terminatingcommunication circuit, said communication channel current detector meansincluding sensing means for detecting the polarity of the supervisorysignal to determine the direction of the supervisory signals in saidcommunication channel.

11. The combination as set forth in claim 9 further including a channelterminating circuit connected to said channel to provide a terminatingcircuit for said supervisory signals, said channel terminating circuitbeing controlled by said channel current dctector and by said 6011-.

trol means so as to be periodically connected to said communicationchannel only during the receipt of supervisory signals on said channel.

12. The combination as set forth in claim 9 further including impulseanalyzer means for detecting the significance of a series of supervisorysignals on said communication channel to generate a third control signalfor activating the supervisory signal gate associated with theterminating communication connection designated by said supervisorysignal thereby connecting said terminating communication connection tosaid communication channel.

13. The combination as set forth in claim 9 further including a channelvoltage detector coupled to said communica-tion channel for providing anadditional control signal when supervisory signals are transmittedsimultaneously from difierent communication circuits.

References Cited UNITED STATES PATENTS 2,917,583 12/1959 Burton et a1.l79l8.9 2,936,338 5/1960 James et a1. 179-l8.9 3,146,314 8/1964 Boehlyet a1. 17918.9 3,205,312 9/1965 Brightman et al. 179-15 3,223,784 12/1965 Inose et al 179-189 KATHLEEN H. CLAFFY, Primary Examiner. L. A.WRIGHT, Assistant Examiner.

1. IN A AUTOMATIC COMMUNICATION SYSTEM; A PLURALITY OF COMMUNICATIONCIRCUITS FOR ORIGINATING AND TERMINATING COMMUNICATION CONNECTIONS, ACOMMUNICATION CHANNEL COMMON TO SAID PLURALITY OF COMMUNICATIONCIRCUITS, A PLURALITY OF INDIVIDUAL GATING MEANS EACH SELECTIVELYCONNECTING A RESPECTIVE COMMUNICATION CIRCUIT TO SAID CHANNEL UPONACTUATION THEREOF, SCANNING MEANS FOR ACTUATING SAID GATING MEANS IN APRESCRIBED SEQUENCE, SUPERVISORY SIGNAL DETECTOR MEANS COUPLED TO SAIDCHANNEL FOR DETECTING THE PRESENCE AND ABSENCE OF SUPERVISORY SIGNALS INTHE FORM OF CURRENT SIGNALS OF GIVEN MAGNITUDE GENERATED BY SAIDCOMMUNICATION CIRCUITS, ANALYZER MEANS CONNECTED TO SAID DETECTOR MEANSFOR INDICATING THE TERMINATING COMMUNICATION CIRCUIT DESIGNATED BY SAIDSUPERVISORY SIGNALS, A TONE SIGNAL CIRCUIT AND CONTROL MEANS RESPONSIVETO SAID DETECTOR MEAND DETECTING ABSENCE OF SAID SUPERVISORY SIGNALS FORCONNECTING SAID TONE SIGNAL CIRCUIT TO THE ASSOCIATED ORIGI-